"Our program is unique, in that we're implementing rural-type solutions in an urban environment," said project leader Thomas "T.H." Culhane, an urban planner and 2009 National Geographic emerging explorer. (The National Geographic Society owns National Geographic News.)

"It's the kind of stuff you would do in the Peace Corps in an African village, but we're doing it right smack dab in the slums of a city."

Solar CITIES's activities are currently limited to Cairo, but the team posts tutorials on how to build and install solar water heaters on YouTube.

Harnessing the Sun

Solar CITIES' hot water systems are constructed from recycled materials and are uniquely tailored to the parts of a city where water and electricity availability are often sporadic.

"The problem with professional solar hot water systems is that they're made for cities with continuous water," Culhane said.

By contrast, Solar CITIES's water heaters use a city's water when it's available but draw from a backup storage tank when it's not.

The setup consists of an insulated rectangular box covered in clear glass or plastic on one side. Inside the box are copper tubes wrapped in sheets of aluminum, which are painted black.

Sunlight striking the darkened aluminum is converted to heat, which is then used to warm water flowing through the pipes. The glass sheet on top of the box prevents the heat from being carried away by wind.